You told us generically that the email address should be authorized to post. Could you be more specific? Is it because s/he is an owner? A subscriber? Specifically, please quote the configuration sections that control posting. Look at the person's subscription. Is "nopost" set?
It used to be possible to have multiple subscriptions with different upper/lower case letters; is this the "case"? ;-)
Finally, consider that Listserv may have different rules for considering minor differences in email addresses significant than your other systems. For example, Google ignores dots in a username; Listserv does not. For example, we have a FirstClass conferencing system where people think of their username as first.last, but when it sends mail to the internet, and Listserv, it uses first_last. As you've checked this it's ot your problem.